Also, being bitter in the way he is is a choice he made.
To no small an extent, the fact that he apparently can't let the next generation find its own path and build on the foundations he laid, nor accept that the young may have a point with their shift in policy regarding the Death Eaters, all of these do not paint a pleasant picture of him.
But we might well see here the core of his grudge; it was not him who taught the young how to fight that second war against Voldemort. That might well be the thing that he hates the most, who so much in canon seems to have centered his sense of self and worth on being an educator, theeducator, of Wizarding Britain. It was someone else who did that, and did so by teaching a very different philosophy that shaped the students than the one Albus wanted to teach.
To no small an extent, the fact that he apparently can't let the next generation find its own path and build on the foundations he laid, nor accept that the young may have a point with their shift in policy regarding the Death Eaters, all of these do not paint a pleasant picture of him.
But we might well see here the core of his grudge; it was not him who taught the young how to fight that second war against Voldemort. That might well be the thing that he hates the most, who so much in canon seems to have centered his sense of self and worth on being an educator, theeducator, of Wizarding Britain. It was someone else who did that, and did so by teaching a very different philosophy that shaped the students than the one Albus wanted to teach.